HGF (hepatocyte growth factor) is a mesenchyme-derived pleiotropic factor which has autogenic, motogenic and morphogenies activities on various different types of cells. The effects of HGF are mediated through its specific tyrosine kinase c-Met. Abnormal expression of HGF and c-Met are frequently observed in many types of cancer, and regulation of HGF/c-Met signaling pathway is known to be involved in tumor growth and metastasis.
Therefore, c-Met is an attractive target for cancer therapy owing to its significant role in the development of cancer. Overexpression of HGF/c-Met is associated with metastasis and angiogenesis of various types of epithelial cells, and from this point of view, a c-Met antagonist antibody targeting c-Met has been proposed as a potential anticancer agent (Comoglio et al., Nature Review Drug Discovery. 7:504-516, 2008). For example, it was reported that HGF-induced c-Met dimerization is negatively regulated by one-armed c-Met antibody in mouse xenograft models, resulting in suppression of tumor growth (Jin et al, Cancer Research 68(11): 4363-4368, 2008; Comoglio et al., Nature Review Drug Discovery. 7:504-516, 2003).
Meanwhile, endocytosis of receptors in response to ligands affects cellular physiology. In other words, receptor trafficking rate determines the duration and propagation of signal transduction and further regulates cell growth, motility and even the fate (Polo et al., Cell, 124(5):897-900, 2006). Therefore, applying the knowledge on molecular mechanisms of endocytosis may give way to control cellular biological processes, and also to provide the effective strategies towards the development of therapeutics.
In this regard, there have been many investigations on the underlying molecular mechanisms of c-Met. For example, it has been known that PKC (Kermorgant et al., EMBO J., 23(19):3721-34, 2004), Cbl (Petrelli et al., Nature, 416(6877):187-93, 2002), dynamin (Dynamin; Li et al., Traffic, 6(6):459-73, 2005), Grb2 (Li et al., J Biol Chem, 282(23):16764-75, 2007) or the like are involved in c-Met endocytosis. However, there have been no reports on the amino acid sequences of c-Met that are essential for endocytosis.
Protein transduction domain (PTD) was first identified during research on the spontaneous entry process of HIV TAT (Green et al., Cell, 55(6):1179-88, 1988). Similar studies were performed on antennapedia, Drosophila homeodomain transcription factor, and they revealed that 16 amino acids spanning from 43 to 58 are required (Derossi et al., J Biol Chem, 271(30):18188-93, 1996). In addition to TAT and antennapedia, VP22 proteins of HSV (Herpes simplex virus) have been also reported to have PTD function (Elliott et. al., Cell, 88:223-233, 1997).